Structurally speaking, being left and pro-open borders is incoherent. The left is about collective power controlling labor supply and using it to tamp down corporate overreach. Open borders and H1B that allow corporations to side-step local worker bargaining power are squarely antithetical to being Left.

I think 'liberal progressives' that are mainly concerned with identity representation, not working class power, are rhetorically bundled as 'far-left'. To people who buy that, Bernie Sanders (a Leftist) and Kamala Harris (a Third Way liberal progressive) are both far-left. It's incoherent but rhetorically potent.